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Part 4 Sheet Silicates Sheet Silicates = Phyllosilicates

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Part 4 Sheet Silicates Sheet Silicates = Phyllosilicates muscovite PART 4 SHEET SILICATES SHEET SILICATES = PHYLLOSILICATES “Phyllon” = leaf Large group of mineral including many common minerals: muscovite, biotite, serpentine, chlorite, talc, clay minerals Structure: interconnected rings (6-fold) of silica tetrahedra that extend outwards infinite sheet: 3 of the 4 oxygen in the silica tetrahedra are shared with other tetrahedra ⇒basic 2- structural unit Si2O5 SHEET SILCATES Most sheet silicates: hydrous Most phyllosilicates: OH- located at the center of the 3- rings ⇒ basic structural unit: Si2O5(OH) Groups: based on structure Two kind of “layers” within the “sheet”: T layers: tetrahedral sites: tetrahedral coordination of Si and Al O layers: octahedral sites: mostly Al and Mg, occasionally Fe T and O layers bounded to form sheet – The space between sheets can be: Vacant Filled with interlayer cations, water or other sheets SHEET SILCATES Different stacking arrangements of tetrahedral sheets and octahedral sheets, along with the type of cation that occupies the octahedral site, allow for the variety of phyllosicilicates that occur in nature. Stacking of the sheets SHEET SILCATES Octahedral sheet variety: Dioctahedral: cations are trivalent (Al3+ or Fe3+) One every three sites is vacant OH and O are bounded to 2 cations Trioctahedral Cations are divalent (Mg2+ or Fe2+) All cation sites are filled OH and O are bounded to 3 cations COMMON SHEET SILCATES T-O Phyllosilicates Si2O5: Dioctahedral: kaolinite Al2Si2O5(OH)4 Trioctahedral:Serpentine (Mg,Fe)3Si2O5(OH)4 T-O-T phyllosilicates Si4O10 Dioctahedral: pyrophyllite Al2Si4O10(OH)2 Trioctahedral: talc (Mg,Fe)3Si4O10(OH)2 T-O-T + interlayer cations: the micas Dioctahedral: muscovite KAl2Si3Al1O10(OH)2 Trioctahedral: biotite K(Mg,Fe)3Si3Al1O10(OH)2 T-O-T + interlayer octahedral sheet: chlorite Di-dioctahedral [Al(OH)3]Al2(Si4)O10(OH)2 Di-Trioctahedral: [Al(OH)3](Mg,Fe)3(Si4)O10(OH)2 Tri-Dioctahedral: [(Mg,Fe)(OH)3]Al2(Si4)O10(OH)2 Tri-Trioctahedral: [(Mg,Fe)(OH)3](Mg,Fe)3(Si4)O10(OH)2 SERPENTINE GROUP (T-O STUCTURE) Trioctahedral T-O Phyllosilicates: (Mg,Fe)3Si2O5(OH)4 3 varieties: Antigorite, Lizardite and Chrisotile Massive – fine grains Fibrous Where? Serpentine = hydration product of Mg-rich silicates (pyroxene, olivine) 2Mg2SiO4 + 3H2O <=> Mg3Si2O5(OH)4 + Mg(OH)2 Olivine water Serpentine Brucite Pseudomorph after olivine and pyroxene in alterated basic and ultrabasic igneous rocks (peridotite, dunite, basalt, gabbro) - often associated with minerals magnesite (MgCO3), chromite, and magnetite. Rock made up almost entirely of serpentine, it is called a serpentinite. TALC (T-O-T STUCTURE) Trioctahedral T-O-T Phyllosilicates: (Mg,Fe)3Si4O10(OH)2 Low hardness Where? low grade metamorphic rocks that originated as ultrabasic to basic igneous rocks. Rock made up almost entirely of talc is called a soapstone. Ex.: Hydrothermal solutions concentrated during final stages of magma crystallization in batholiths or hot seawater solutions drawn down into subduction zones Use: - in paint, plastics, rubber, various roofing compounds: as lubricant - in cosmetic and body powder. - as “whiskistone” MICA GROUP (T-O-T + C STUCTURE) White micas (dioctahedral) vs. black micas (trioctahedral) Muscovite, Paragonite, and Margarite Biotite and Clintonite White micas: Muscovite, KAl3Si3O10(OH)2, and Paragonite, NaAl3Si3O10(OH)2: end members of the solid solution series involving K and Na but large immiscibility gap (solvus): Muscovite = 65 to 100% K vs paragonite = 80 to 100% Na Where? Al-rich medium grade metamorphic rocks (Al-rich schist) in siliceous, Al-rich plutonic igneous rocks (but not in volcanic rock) - association with alkali feldspar, quartz, and sometimes biotite, garnet, andalusite, sillimanite, or kyanite. Substitution of Al by Li in the interlayer cation of muscovite: Lepidolite – pink mica found in pegmatite MICA GROUP (T-O-T + C STUCTURE) White micas (dioctahedral) vs. black micas (trioctahedral) Muscovite, Paragonite, and Margarite Biotite and Clintonite Black micas Biotite is a solid solution between the end members Phlogopite KMg3AlSi3O10(OH)2 and Annite , KFe3AlSi3O10(OH)2 (but pure Annite does not occur in nature) Substitutions: - minor Na, Rb, Cs, and Ba may substitute for K - minor F can substitute for OH: increase the stability at high temperature Where? Phlogopite: in hydrous ultrabasic volcanic rocks (kimberlite), in metamorphosed dolomite Biotite: dacitic, rhyolitic, and trachytic volcanic rocks, granitic plutonic rocks, and a wide variety of metamorphic rocks. CHLORITE (T-O-T + O LAYER) structure that consists of phlogopite T-O-T layers sandwiching brucite-like octahedral layer. Substitutions: - Mg for Fe - Al for (Mg, Fe) in both the octahedral sites - Al for Si Where? low grade metamorphic rocks – associated with actinolite, epidote, and biotite. It also forms as an alteration product of pyroxenes, amphiboles, biotite, and garnet in igneous as well a metamorphic rocks. CLAY MINERALS Products of chemical weathering – main constituent of mudrock (mudstone, claystone, shale) 40% minerals in sedimentary rocks Understanding their behavior: important economic use (ceramic), civil engineering (clay swelling) Structural classification: Kandites (struc. ⇔ Kaolinite) Smectites (struc. ⇔ Pyrophyllite) Illites (struc. ⇔ muscovite) T-O T-O-T T-O-T+c CLAY MINERALS Chemical Weathering: Minerals form in depth are not stable at the surface of Earth: Lower T (-20 to 50°C) Lower P (1 to few hundred atmospheres) Higher free water Higher free O Stability order at the near surface conditions: Iron oxide – Aluminium oxide, quartz, clay mineral, Muscovite, Alkali feldspar, biotite, Amphibole, Pyroxenes, Ca-rich plagioclase, Olivine CLAY MINERALS Chemical Weathering – Main agent: water and weak acids in water Acid in solution: abundant free H+ Most common acid: Carbonic acid: produced by reaction between rainwater and carbon dioxide in atmosphere: + - H2O + CO2 → H2CO3 → H + HCO3 Water carbon dioxide carbonic acid hydrogen ion bicarbonate ion CLAY MINERALS Type of chemical weathering reactions: Hydrolysis: H+ or OH- replaces an ion in the mineral + + 4KAlSi3O8 + 4H + 2H2O → 4K + Al4Si4O10(OH)8 + 8SiO2 Orthoclase hydrogen ion water potassium ion kaolinite quartz 2+ 3+ Oxidation: reaction of mineral with O2: change of the oxidation state (Fe to Fe ) 2+ 3Fe SiO3 + 1/2O2 → Fe3O4 + 3SiO2 Pyroxene oxygen magnetite quartz Dehydration – Removal of H2O or OH- ion from a mineral 2FeO·OH → Fe2O3 + H2O Goethite hematite water Complete dissolution (in water) CLAY MINERALS Kandites (T-O structure) Most common: kaolinite Al2Si2O5(OH)4 Other kandites: Anauxite, Dickite, Nacrite Where? Weathering of hydrothermal alteration of aluminosilicate minerals ⇒ feldspar-rich rocks (ex.: granitic rocks) weather to kaolinite Smectites (T-O-T structure) Structure similar to pyrophyllite Al2Si4O10(OH)2 Possible substitution of Al with Mg or Fe ⇒ di- and trioctahedral Possibility to absorb water between the TOT sheets: cause volume of the mineral to increase in contact with water : expanding clays Most common: Montmorillinite . (½Ca,Na)(Al,Mg,Fe)4(Si,Al)8O20(OH)4 nH2O = main constituent of Bentonite, derived by weathering of volcanic ash. CLAY MINERALS Illites (T-O-T+c structure) KyAl4(Si8-y,Aly)O20(OH)4 usually with 1 < y < 1.5, but always with y < 2. Possible substitution of K by Ca or Mg (to preserve balance) The most common in soils. Not an expanding clay Where? formed from weathering of K and Al-rich rocks under high pH conditions ⇔ by alteration of minerals like muscovite and feldspar. Main constituent of ancient mudrocks and shales. Mixed layer clays = change from one type to another through a stacking sequence Common, regular and ordered or irregular and unordered Identification? To thin/small to be recognize in hand samples or petrographic microscope ⇒ use of the X-ray technique .
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